1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to an organic electroluminescent display (OELD) device and a method of driving the same.
2. Discussion of the Related Art
Until recently, display devices have typically used cathode-ray tubes (CRTs). Presently, many efforts and studies are being made to develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays, and electro-luminescence displays (ELDs), as a substitute for CRTs. Of these flat panel displays, organic electroluminescent display (OELD) devices are self-luminescent display devices. The OELD devices operate at low voltages and have a thin profile. Further, the OELD devices have fast response time, high brightness, and wide viewing angles.
FIG. 1 is a circuit diagram illustrating a sub-pixel of an OELD panel according to the related art, and FIG. 2 is a wave form of voltages driving the sub-pixel of FIG. 1.
Referring to FIG. 1, the OELD device includes a gate line S and a data line D to define a sub-pixel. The sub-pixel includes a switching transistor SW, a driving transistor DR, a storage capacitor C, and an organic light emitting diode OLED. Gate and source of the switching transistor SW are connected to the gate and data lines S and D, respectively. A gate of the driving transistor DR is connected to a drain of the switching transistor SW, a drain of the driving transistor DR is connected to a cathode of the organic light emitting diode OLED. An anode of the organic light emitting diode OLED is applied with a first driving voltage VDD. A source of the driving transistor DR is connected to a second driving voltage VSS. The second driving voltage VSS may be lower than the first driving voltage VDD and be a ground voltage. A storage capacitor C is connected to both of the gate and source of the driving transistor DR. Each of the switching and driving transistors SW and DR may be a negative type and include an amorphous silicon layer.
Referring to FIGS. 1 and 2, when the gate voltage has an on level, for example, a high level VGH, the switching transistor SW is turned on. When the switching transistor SW is turned on, a data voltage Vdata is applied to the gate of the driving transistor DR and stored in the capacitor C. An amount of the data voltage Vdata determines an amount of a current applied to the organic light emitting diode OLED, and the amount of the current determines an amount of a light emitted from the organic light emitting diode OLED. In other words, the data voltage Vdata determines brightness of the emitted light.
However, since the driving thin film transistor DR uses the amorphous silicon, an electrical property, for example, a mobility of the thin film transistor Dr may be varied due to surroundings such as a temperature and an ambient light.
FIG. 3A is a graph illustrating variation of a panel current of the related art OELD panel according to variation of a temperature, and FIG. 3B is a graph illustrating variation of a panel current of the related art OELD panel according to exposure of an ambient light. In FIGS. 3A and 3B, the panel current is a total of currents applied to all organic light emitting diodes (OLED of FIG. 1) in the OELD panel.
Referring to FIG. 3A, during periods B and D, a cooling fan is operated to cool the driving transistors (DR of FIG. 1) in the OELD panel. Accordingly, the panel current during the periods B and D is thus lowered compared to the panel current during periods A, C and F when the cooling fan is not operated. In other words, according to variation of a temperature, the panel current of the OELD panel is greatly varied.
Referring to FIG. 3B, during a period B, an ambient light is incident on the driving transistors in the OELD panel and this causes a photo-leakage in the driving transistors. Accordingly, the panel current during the period B increases compared to the panel current during periods A and C when the ambient light is not incident on the OELD panel and the photo-leakage does not occur. In other words, according to exposure of an ambient light, the panel current of the OELD panel is greatly varied.
Such the variation of the panel current of the OELD panel due to the surroundings causes variation of brightness in displaying images. Accordingly, display quality and reliability is degraded.